One observer wrote in the Peterborough Chronicle: “On the fifth night in the month of May appeared the Moon shining bright in the evening, and afterwards by little and little its light diminished, so that, as soon as night came, it was so completely extinguished withal, that neither light, nor orb, nor anything at all of it was seen.” For almost 1,000 years, astronomers have attempted to solve the mystery of the extremely dark eclipse, but to no avail. However, by analysing ice cores – samples taken from deep within ice sheets or glaciers, which have trapped sulphur aerosols – researchers believe they finally have the answer. In the years before the vanishing Moon phenomenon, there were a series of volcanic eruptions around the globe which pumped dust and sulphur into the atmosphere. Some assumed it was the sulfurous deposit left by a major 1104 eruption of Iceland’s Hekla volcano that was to blame. However, another more recent research concluded that a timescale named the Greenland Ice Core Chronology 2005 (GICC05) was showing the wrong dates for some of the events. The GICC05 information is what lead the new research team led by Sébastien Guillet from the University of Geneva to deduce that it could not have been the Hekla volcano’s eruption that led to that specific phenomenon. The team then looked into medieval records that described dark lunar eclipses that could correspond to this event. To investigate what might have been responsible for leaving these ancient tracks at both the top and the bottom of the world, the team combed historical documentation, looking for medieval records of strange, dark-looking lunar eclipses that could correspond to the stratospheric haze of major eruptive events. “The spectacular atmospheric optical phenomena associated with high-altitude volcanic aerosols have caught the attention of chroniclers since ancient times,” the team writes. “In particular, the reported brightness of lunar eclipses can be employed both to detect volcanic aerosols in the stratosphere and to quantify stratospheric optical depths following large eruptions.” According to NASA records based on astronomical retrocalculation, seven total lunar eclipses would have been observable in Europe in the first 20 years of the last millennium, between 1100 and 1120 CE. Among these, a witness to a lunar eclipse that occurred in May 1110 wrote of the exceptional darkness of the Moon during the phenomenon. “On the fifth night in the month of May appeared the Moon shining bright in the evening, and afterwards by little and little its light diminished, so that, as soon as night came, it was so completely extinguished withal, that neither light, nor orb, nor anything at all of it was seen,” an observer wrote in the Peterborough Chronicle. Many astronomers have since discussed this mysterious and unusually dark lunar eclipse. Centuries after it occurred, the English astronomer Georges Frederick Chambers wrote about it, saying: “It is evident that this [eclipse] was an instance of a ‘black’ eclipse when the Moon becomes quite invisible instead of shining with the familiar coppery hue”. Despite the event being well-known in astronomy history, though, researchers have never suggested it might have been caused by the presence of volcanic aerosols in the stratosphere, even though that’s the most likely cause, the new study suggests. “We note that no other evidence of volcanic dust veil, such as a dimming of the Sun, red twilight glows and/or reddish solar haloes, could be found during our investigations for the years 1108–1110 CE,” the researchers write. If the timing is right, then what volcano was responsible for the sulphur cloud, given Hekla is now out of the frame? While it’s impossible to know for sure, the team thinks the most probable explanation is Japan’s Mount Asama, which produced a giant, months-long eruption in the year 1108 – significantly larger than a subsequent eruption in 1783 that killed over 1,400 people. A diary entry recorded by a statesman describes the 1108 event: “There was a fire at the top of the volcano, a thick layer of ash in the governor’s garden, everywhere the fields and the rice fields are rendered unfit for cultivation. We never saw that in the country. It is a very strange and rare thing.” In addition to witness accounts, the researchers also looked at tree ring evidence, which suggests 1109 CE was an exceptionally cold year (about 1 degree Celsius cooler in the Northern Hemisphere), based on significantly thinner tree rings. Other historical documentation, in particular accounts of climatic and societal impacts in the years 1109–1111 CE, corroborate the hypothesis that an 1108 eruption (or a series of eruptions that began that year), could have led to disastrous effects on affected communities. The researchers found an “abundance of testimonies referring to adverse weather, crop failures, and famines in these years”, noting that the “assembled evidence suggests that the subsistence difficulties, which began in 1109, deepened into famine in several regions of western Europe”. The new research is a reminder that our planet, and its civilizations, are deeply interconnected. A natural disaster in one corner of the world can throw communities thousands of miles away into turmoil, and can even darken the Moon on a clear night.